How to add a "select path" input in a UI addon script?
Well, I finally found a solution:

First, I had to define a custom property in the register() function:

def register():
    bpy.utils.register_module(__name__)
    bpy.types.Scene.conf_path = bpy.props.StringProperty \
      (
      name = "Root Path",
      default = "",
      description = "Define the root path of the project",
      subtype = 'DIR_PATH'
      )

def unregister():
    bpy.utils.unregister_module(__name__)
    del bpy.types.Scene.conf_path


Then in the draw() method of the panel class, I had to call the property this way:

def draw(self, context):
    layout = self.layout
    col = layout.column()
    col.prop(context.scene, 'conf_path')


And... it worked!

Now, I wanted to display the default value (which is stored in a separate ".conf" file), in the "file input" when Blender starts (or the add-on is enabled).
To do so, I just had to call a function which get this value, and put it in the "default" parameter of the property definition:

def register():
    default_path = get_path_from_config_file()
    bpy.utils.register_module(__name__)
    bpy.types.Scene.conf_path = bpy.props.StringProperty \
      (
      name = "Root Path",
      default = default_path,
      description = "Define the root path of the project",
      subtype = 'DIR_PATH'
      )


And that's it!